A vehicle suspension increases passenger comfort and improves vehicle handling by absorbing the impact of road imperfections, wind and vehicle actions, such as braking, acceleration and turning. The most basic elements of the suspension are the springs that support the vehicle chassis, motor and cab (known as the “the sprung masses”) over the wheels and wheel components (known as “the unsprung masses”). However, springs permit or create oscillations, vibrations, response overshoots and other undesirable motions in the sprung masses.
Dampers, also known as shock absorbers, are commonly employed to further improve the handling and ride of the vehicle by reducing such undesirable motions by absorbing and dissipating a portion of the kinetic energy that would otherwise flow through the springs. For example, after traveling over a bump in the road, the sprung masses tend to oscillate. A damper will allow the suspension to diffuse the impact of the bump, but will reduce the tendency for the sprung masses to oscillate thereafter.
Dampers have two ends, one mechanically connected to the sprung masses and the other to the unsprung masses. Dampers are able to elongate and contract, to accommodate the varying distances between the sprung and unsprung masses during jounce or rebound phase. The rate of damper contraction or elongation is referred to as the damper's relative velocity. The extension of the damper between the sprung and unsprung masses dampers is known as the damper's relative displacement.
Dampers are categorized as passive, semi-active or active. For a passive damper, the damping force—the force of the dampers' resistance to extension or contraction—will always be the same for a given relative velocity, as defined by the hydraulic or mechanical interactions of its parts. In other words, the passive damper has a fixed damping force-relative velocity relationship. A semi-active damper has a variable damping force-relative velocity relationship which allows it to adapt to certain operating conditions. An active damper is capable of powering, as opposed to merely resisting, a relative velocity in the damper.
Conventional dampers work by converting the kinetic energy of undesirable vehicle motion to heat. Dampers made of solid elements dissipate the kinetic energy as heat generated by friction; newer hydraulic dampers dissipate the kinetic energy as heat generated by turbulent or viscous flow. More recent designs based on intelligent fluids such as electro—and magneto-rheological fluids also dissipate the kinetic energy as heat resulting from turbulent or viscous flow.
The energy dissipated by the dampers reduces vehicle efficiency. For example, damping forces can account for approximately 15% of the total energy expenditure for a compact car traveling at 45 miles per hour. This is a significant energy drain for the vehicle, especially in light of the increasing importance of vehicular energy efficiency, both for marketing purposes and compliance with governmental regulations.
Harnessing this otherwise wasted energy is one approach to improving vehicle efficiency. However, the concept of power regeneration from dampers has not been significantly developed. Some existing regenerative dampers are unable to contribute to overall vehicle efficiency. Other regenerative dampers are difficult to integrate with conventional suspension systems due to their complexity. Yet others have not been adapted for use in vehicles.